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After thermal treatment at 120 °C in the presence of hydrochloric acid (thermal-acid treatment), soybean protein increased its water-insoluble network structure due to the molecular rearrangement and aggregation of unfolded protein molecules, which improved its water resistance. Modification with various crosslinkers, namely glyoxal, polyisocyanate, a glyoxal–polyisocyanate combination, waterborne epoxy latex and modified polyamide, were employed to further enhance the water resistance of the thermal-acid treated soybean protein (TSP). FTIR and DSC analyses confirmed that all of the crosslinkers could chemically react with the TSP, but the crosslinking degree varied with crosslinker species. The positive effect of crosslinker species on the water resistance and thermal stability of the TSP as evaluated by a boiling water-insoluble test and TGA analysis was in the order modified polyamide > glyoxal–polyisocyanate > polyisocyanate > epoxy latex > glyoxal. In addition, wood flour-reinforced soybean protein-based composites were successfully fabricated to investigate the possibility of manufacturing acceptable biocomposites using crosslinked and thermal-acid treated soybean protein. FTIR analysis, SEM observation and tensile property evaluation showed that all crosslinkers except glyoxal could react with wood flour to form various chemical bonds at the protein–wood interface and therefore significantly improve the water resistance and tensile strength of the obtained composites. The extent of improvement depended on the crosslinker species and crosslinking mechanism. Modified polyamide was the most preferable crosslinker for commercial applications because its modified composite had good tensile strength and the best water resistance. [ABSTRACT FROM AUTHOR]

In order to extend the applications of wood composites and products bonded by soybean protein adhesive from interior to exterior fields of application, this study proposes a novel approach for improving the water resistance of soybean protein-based wood adhesives using thermo-chemical treatment of soybean protein. The soybean protein formed stable three-dimensional networks due to repolymerization or self-crosslinking during thermo-chemical treatment, confirmed by both increases in the water-insoluble content of the treated soybean protein and the improved hydrothermal-aged wet bond strength of the resulting soybean protein adhesive. Thermo-chemical treatment in the presence of 1 wt% sodium sulfite (which cleaves disulfide bonds) and 1 wt% sodium dodecyl sulfate (which destroys the hydrophobic interactions of proteins) released active groups buried within the globular structure of soybean protein via unfolding. This release both promoted the repolymerization of the soybean protein molecules and exposed more active sites for effective crosslinking by the post-added crosslinker EMPA. Plywood bonded by the optimal soybean protein adhesive possessed a good hydrothermal-aged bond strength of 1.22 MPa, exceeding the value required for structural use according to the JIS K6806-2003 commercial standard. [ABSTRACT FROM AUTHOR]